1. Field of the Invention
The present invention relates generally to a variable displacement vane-type rotary comressor. More specifically, the present invention relates to a variable displacement vane-type rotary compressor to be used as a refrigerant compressor for an air conditioner of a vehicle.
2. Description of the Background Art
In a variable displacement vane-type rotary compressor, a front plate fixedly closing a front end of a cam ring is formed with a pair of induction ports and a pair of bypass ports and an adjust plate formed with a corresponding pair of bypass openings is rotatably fitted into a central circular recess formed on the rear side of the front plate. A rotational displacement of the adjust plate varies a position of each bypass opening relative to the corresponding induction port and bypass port so as to control a compression starting point of a rotary vane in a working chamber provided in the cam ring. This type of the variable displacement vane-type rotary compressor is disclosed, for example, in a First Japanese Patent Publication No. 63-41692.
FIG. 1 shows a front plate and an adjust plate which are used in such a rotary compressor. A disk-shaped front plate 2 is formed at its center with a circular recess 4. The circular recess 4 is formed at its bottom with a pair of induction ports 6 located in a rotation symmetry with respect to a rotation axis of a rotor or an axis of the compressor, and with a pair of bypass ports 8 located in a rotation symmetry with respect to the rotation axis of the rotor. Each induction port 6 includes a recessed portion 10 which is formed by cutting out a portion of the circumferential wall 12 in a manner to enlarge dimensions of the opening, formed through the bottom of the circular recess 4, of the induction port 6. The bypass port 8 is located by spacing a predetermined distance from the induction port 6 in a direction along the rotation of the rotor, as indicated by an arrow in FIG. 1. Each induction port 6 and the corresponding bypass port 8 are arranged such that when a leading edge, with respect to the rotational direction of the rotor, of a sickle-shaped working chamber formed in the cam ring matches a leading end 14 of the induction port 6, a trailing end 16, with respect to the rotational direction of the rotor, of the bypass port 8 is located in the vicinity of a discharge port provided at a trailing end of the sickle-shaped working chamber.
The adjust plate 18 is rotatably fitted into the circular recess 4 with its circumferential periphery being in slidable contact with the circumferential wall 12 and with its front surface being in slidable contact with the bottom of the circular recess 4. In this condition, a rear surface of the adjust plate 18 is on a level with a rear annular surface of the front plate 2.
A pair of bypass openings 20, in the form of recessed cut-outs, are formed on the circumferential periphery of the adjust plate 18. The bypass openings 20 are located in a rotation symmetry with respect to the rotational axis of the rotor. Each bypass opening is of a size similar to that of the corresponding induction port 6.
A rotational displacement of the adjust plate 18 is controlled by control means provided in the compressor to vary a position of each bypass opening 20 relative to the corresponding induction port 6 and bypass port 8 so as to adjust a compression starting point of the rotary vane within the sickle-shaped working chamber. As shown in FIG. 2(A), the compression starting point is most advanced to maximize its discharge when the induction port 6 and the bypass opening 20 coincide with each other, i.e. the bypass opening 20 coincide with each other, i.e. the bypass opening 20 is only in communication with the induction port 6 and not in communication with the bypass port 8. This is because no working refrigerant which is introduced in to the sickle-shaped working chamber from an induction chamber through the induction port 6 and the bypass opening 20, is returned into the induction chamber through the bypass port 8. In this condition, the rotational displacement of the adjust plate is minimum. As shown in FIG. 2(B), the compression starting point is between most advanced and most retarded to make its discharge intermediate when a leading end 22, with respect to the rotational direction of the rotor, of the bypass opening 20 exceeds a trailing end 24 of the induction port 6 by predetermined distances in the rotational direction of the rotor. This is because a portion of the working refrigerant introduced through the recessed portion 10 of the induction port 6 is returned into the induction chamber through the bypass opening 20 and the bypass port 8, and the compression by the rotary vane starts after the rotary vane reaches a trailing end 26 of the bypass opening 20. In this condition, the rotational displacement of the adjust plate 18 is intermediate. As shown in FIG. 2(C), the compression starting point is most retarded to minimize its discharge when the trailing end 26 of the bypass opening 20 substantially coincide with the trailing end 16 of the bypass port 8. This is because most of the working refrigerant introduced through the recessed portion 10 of the induction port 6 is returned into the induction chamber through the bypass opening 20 and the bypass port 8, and the compression by the rotary vane starts after the rotary vane reaches the trailing end 26 of the bypass opening 20, which is close to the discharge port. In this condition, the rotational displacement of the adjust plate 18 is maximum.
The structure described above, however, involves the following problems.
In FIG. 2(a) where the compression starting point is most advanced, since a sufficient amount of the working refrigerant is introduced into the working chamber through the matched induction port 6 and bypass opening 20, no serious questions is raised. However, in FIGS. 2 (B) and (C), since the working refrigerant is introduced into the working chamber only through the recessed portion 10 of the induction port 6, the induction amount of the working refrigerant is insufficient to cause the power loss due to the pressure differential between forward and rearward of the rotary vane in the rotational direction of the rotor. Further, in FIGS. 2 (B) and (C), when the rotary vane is located between the recessed portion 10 of the induction port 6 and the bypass opening 20, the vane defines two sections in the working chamber forward and rearward of the vane, which are discommunicated with each other. Accordingly, the working refrigerant first introduced into the working chamber from the induction chamber through the recessed portion 10 of the induction port 6 is returned into the induction chamber through the bypass opening 20 and the bypass port 8, and is again introduced into the working chamber through the recessed portion 10. This recirculation of the working refrigerant causes agitation of the working refrigerant to increase a temperature thereof. This induces a lowering of durability of the compressor.